Wavelength division multiplexing (WDM) is widely put to practical use as one technology for increasing capacity of an optical communication network. In a WDM transmission system, a WDM signal is generated by multiplexing a plurality of wavelength channels. Data signals are transmitted through respective wavelength channels in the WDM signal. The WDM signal is arranged in a wavelength band called the C-band (approximately 1530 to 1565 nm), for example.
In order to further increase the capacity of the WDM transmission system, a configuration in which the WDM signal is arranged also in the L-band and/or the S-band in addition to the C-band is proposed. The wavelength of the L-band is longer than that of the C-band, and the wavelength of the S-band is shorter than that of the C-band.
However, a configuration for transmitting the WDM signal using the L-band or the S-band as compared with the C-band is not widespread. For that reason, an optical transceiver for the L-band or S-band is currently expensive. On the other hand, development of a wavelength converter that converts a wavelength of an optical signal between the C-band and another band (L-band or S-band) is proceeding. Accordingly, a large capacity transmission system in which WDM signals are transmitted using the C-band, the L-band, and the S-band, respectively, by combining the optical transceiver for C-band and the wavelength converter for wavelength conversion between the C-band and the other band is proposed.
In Japanese Laid-open Patent Publication No. 2003-188830, a configuration in which a wavelength multiplexed optical signal having a plurality of wavelength bands is amplified by using a predetermined type of optical amplifier for a single wavelength band is proposed.
In Japanese Patent No. 4550187, a method for converting a wavelength using an optical fiber is disclosed.
In Japanese Laid-open Patent Publication Nos. 2014-165543 and 2016-208310, a method for detecting or adjusting power of each wavelength of the WDM signal is disclosed.
However, conversion efficiency of the wavelength converter has wavelength dependency. For that reason, for example, even when a WDM signal of which power of a plurality of wavelength channels is uniform is generated, the power of the plurality of wavelength channels in the WDM signal output from the wavelength converter may not be uniform. In this case, transmission performance (optical signal-to-noise ratio (OSNR), bit error rate (BER), and the like) varies for each wavelength channel.
Accordingly, in an optical transmission system that transmits a plurality of WDM signals having different wavelengths by using wavelength conversion, monitoring each WDM signal to control the characteristics of each WDM signal is demanded. However, when a plurality of WDM signals having different wavelengths are monitored, a plurality of different monitor devices are demanded, which may increase the cost for the optical transmission system.
In view of the matters described above, in the optical transmission system that transmits a plurality of wavelength division multiplexed optical signals using a plurality of wavelength bands, it is desirable to control the characteristics of each wavelength division multiplexed optical signal by monitoring the wavelength division multiplexed optical signal at one wavelength band.